Ionisation induced collapse of minihaloes

Download

Date

Author

Metadata

Abstract

The first stars, galaxies and black holes in the universe produced large
quantities of ionising UV radiation; forming H II regions in the neutral gas
before the Epoch of Reionisation (EoR). These ionisation fronts will have come
into contact with overdensities in the surrounding Intergalactic Medium (IGM),
including haloes which were in the process of collapse. Previous studies have
shown that the feedback processes on these secondary haloes can either disrupt
the gas, or induce further cooling from the formation of molecular hydrogen. The
ionising source will eventually die and create a defunct H II region, which expands
into the surrounding neutral IGM. Minihaloes at the edge of these defunct H II
regions are particularly susceptible to positive feedback due to not having been
photoevaporated like their counterparts further inside the ionised volume.
In this thesis, numerical simulations of minihaloes at the edges of H II
regions formed by the first luminous objects before the EoR are presented. A
methodology of including secondary ionisations from high energy photons is also
implemented into the radiation hydrodynamical code ZEUS-RT.
The interaction of differing spectral index sources with minihaloes including
secondary ionisation is discussed. The secondary ionisations show the greatest
effect for hard spectral sources with a large fraction of high energy photons; where
a decrease in photoheating and an increase in ionisation rate is found at the outer
reaches of the ionisation front (I-front). The increased ionisation rate lowers the
optical depth of the gas for subsequent photons, and thus reduces the trapping
of I-fronts in high densities found in the minihalo cores. The ratio of the free
electron fraction to the temperature in the core of the minihaloes is found to
constrain the resulting evolution. A high ratio is correlated with the creation of
molecular hydrogen, which can then induce further cooling and the collapse of
the system.A large parameter suite of 3780 two-dimensional minihalo models utilising
radiative hydrodynamical simulations with 12 species and a coupled reaction
network, including H2 cooling, HD cooling, Lyman-Werner radiation and secondary
ionisation is performed. The parameter space includes: the spectral
index representing different sources such as quasars or galaxies, the mass of the
minihaloes from 105 - 106 Mʘ, the redshift of ionisation from z ~ 10 - 30, and
other factors which denote the position of the minihalo relative to the boundary
of the H II region. Minihaloes with average core densities of n0 = 2 - 10 cm-3
show almost unanimous positive feedback, while the majority of minihaloes under
this average density are disrupted. Minihaloes with core densities above this
value generally would have collapsed in isolation anyway, but are found to not
be delayed by the I-front. The H2 fraction in the minihalo gas is also increased
in models with no blowout by factors between 2 - 100 times that of an isolated
minihalo. This is especially significant for lower redshift, z ≤ 15, minihaloes.
Finally, a parameter suite of larger 106 - 107 Mʘ minihaloes results in the
creation of self-gravitating clumps of gas moving out of the dark matter potential.
The gas core is compressed by the I-front, enriched with molecular hydrogen, and
ejected by the pressure front after the source dies. These "baryon bullets" could
be progenitors of primordial globular clusters found in the haloes of galaxies
today. Properties such as old stellar populations and the lack of dark matter
haloes can be explained by this radiative ejection method.
The dynamical nature of these interacting systems and diversity of subsequent
evolution suggest that minihaloes less than 108 Mʘ are important in the
early formation history of the universe, and in determining the constraining
parameters of the EoR. The feedback mechanisms investigated, and secondary
ionisation physics, should be included in astrophysical simulations and analytical
calculations determining the evolution of the universe at this critical epoch.